1. Field of the Invention
This invention generally relates to serrated Fourier filters and inspection systems. Certain embodiments relate to a Fourier filter that includes periodic serrations, which are formed on edges of one or more blocking elements, and which are configured to increase smoothness of transmission variations across a transition region of the one or more blocking elements.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the device to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
Many different types of inspection tools have been developed for the inspection of semiconductor wafers. The inspection tools may be categorized generally according to the types of specimen that they are designed to inspect. For example, one category of inspection tools is generally designed to inspect unpatterned semiconductor wafers. Since these tools are optimized for inspecting unpatterned wafers, these tools are generally not capable of inspecting patterned wafers for a number of reasons. For example, many unpatterned wafer inspection tools are configured such that all of the light collected by a lens or another collector is directed to a single detector that generates a single output signal representative of all of the light collected by the lens. Therefore, light scattered from patterns or other features on the specimen will be combined with other scattered light. As such, the single detector may become saturated and consequently will not yield signals that can be analyzed for defect detection. In addition, even if the single detector does not become saturated, the light scattered from patterns or other features on the wafer can not be separated from other scattered light thereby hindering, if not preventing, defect detection based on the other scattered light.
Patterned wafer inspection is of particular interest and importance to the semiconductor industry because processed semiconductor wafers usually have a pattern of features formed thereon. Although inspection of unpatterned wafers, or “monitor wafers,” which have been run through a process tool, may be used as a gauge for the number and types of defects that may be found on patterned wafers, or “product wafers,” defects detected on monitor wafers do not always accurately reflect the defects that are detected on patterned wafers after the same process in the process tool. Inspection of patterned wafers after such processing is, therefore, important to accurately detect defects that may have been formed on the wafer during, or as a result of, processing. Therefore, inspecting patterned wafers or product wafers may provide more accurate monitoring and control of processes and process tools than inspection of monitor wafers.
Many inspection tools have been developed for patterned wafer inspection. For example, one patterned wafer inspection tool utilizes spatial filters to separate light scattered from patterned features from other scattered light such that the other scattered light may be separately detected. Since the light scattered from patterned features depends on various characteristics of the patterned features such as lateral dimension and period, the design of the spatial filter also depends on such characteristics of the patterned features. As a result, the spatial filter must be designed based on known or determined characteristics of the patterned features and must vary as different patterned features are being inspected.
One type of spatial filter that may be used as described above is a Fourier filter. Fourier filters are relatively useful for filtering light from patterned features formed on a wafer. However, Fourier filters can have adverse effects on the transmitted light because the filters can diffract desirable light into undesirable directions. To reduce the adverse effects of a Fourier filter on the transmitted light, the transition between the blocking area and the transparent area of the Fourier filter should be relatively smooth without introducing phase change to the transmitted light.
A number of different approaches have been proposed to increase the smoothness of the transition from the blocking to transparent areas of Fourier filters. One approach is to form a blocking area of a Fourier filter on a glass plate with 100% opaqueness at the center of the blocking area with a gradual transition to 100% transmission at the edges. This kind of Fourier filter can perform well as long as it is made correctly. However, fabricating this kind of filter is not only expensive but also technically challenging. Another approach is to introduce random serrations on the edges of otherwise 100% opaque blocking elements of a Fourier filter. Fourier filters made in this manner are relatively inexpensive but do not provide the desired performance.
Accordingly, it would be advantageous to develop a Fourier filter that has a relatively smooth transition between blocking and transmission areas thereby providing relatively good performance and that is relatively inexpensive and easy, technically speaking, to manufacture.